Minerals, digestion methods

Dehairs, F., Decadt, G. and Baeyens, W. (1982) Comparative study of different wet mineralization digestion methods for the measurement of total mercury in biological samples. Analusis, 10, 373-376. 

The subsamples were split and sent to different laboratories to be subjected to ten commonly-used and proprietary leach/digestion techniques (a) aqua regia partial digestion method at Acme Analytical Laboratories (b) sodium pyrophosphate and cold hydroxylamine leaches at ALS Chemex (c) enzyme and TerraSol leach methods at Skyline Labs (d) Bioleach and soil gas hydrocarbon analyses at Activation Laboratories (e) Mobile metal ion (MMI) extraction at SGS Minerals (f) 4-acid near-total and sodium peroxide sinter total digestions (under the uses contract) at SGS Minerals and (g) de-ionized water leach at the USGS laboratories. 

Fusion decomposition provides an alternative digestion method. This is a high-temperature technique where powdered samples are heated with a suitable flux to produce a residue that may be readily dissolved. Fusion decompositions are the most rigorous digestions available and all silicate materials can be brought into a complete solution when fused with an appropriate flux. The principal disadvantage of the fusion technique is the introduction of extra salts into the final solution from the flux (thereby increasing total dissolved solids). Fusion decomposition remains the preferred technique for quantitative analysis of silicon, and may be the only practical method for complete decomposition of refractory minerals such as zircon, rutile, and cassiterite. Many fluxes have been used, but perhaps the most commonly used is lithium metaborate (LiBOs). This flux introduces only Li and B into the final solution, and is used at a relatively low flux sample ratio of 3 1. 

Precondition for a voltammetric determination of trace elements in urine is a sample pretreatment. The problem of losses of mercury with classic mineralization methods is described above. To avoid this problem, the authors have developed an acid digestion method with the addition of thioacetamide. All mercury in the sample is converted by this to the extremely poorly soluble mercury sulfide (HgS), which is stable under the conditions chosen. Subsequently hydrogen peroxide is added to oxidize the HgS and liberate the Hg ion [113]. 

Two methods of sample preparation were investigated. The former is dilution of blood semm with 0.1% Triton X-100, the latter is aeid mierowave digestion. As evaluated, the most adequate mineralization proeedure for determining the majority of elements in blood semm by ICP AES is aeid mierowave digestion. However, the ICP AES determination of abundant elements (B, Si, Mn), whieh present in semm at 0.001-0.01 ppm levels should be follow sample dilution with Triton X-100. 

Yttrium and lanthanum are both obtained from lanthanide minerals and the method of extraction depends on the particular mineral involved. Digestions with hydrochloric acid, sulfuric acid, or caustic soda are all used to extract the mixture of metal salts. Prior to the Second World War the separation of these mixtures was effected by fractional crystallizations, sometimes numbered in their thousands. However, during the period 1940-45 the main interest in separating these elements was in order to purify and characterize them more fully. The realization that they are also major constituents of the products of nuclear fission effected a dramatic sharpening of interest in the USA. As a result, ion-exchange techniques were developed and, together with selective complexation and solvent extraction, these have now completely supplanted the older methods of separation (p. 1228). In cases where the free metals are required, reduction of the trifluorides with metallic calcium can be used. 

We have developed an in vitro digestion procedure, not as a substitute for in vivo studies, but as a useful adjunct. Our initial objective was to develop an in vitro procedure for measuring exchangeability, the fraction of the food mineral which exchanges with an extrinsic isotope tracer added to the food. This was expected to facilitate the measurement of food mineral absorption in humans by the extrinsic tag method. Secondary objectives were to determine if in vitro mineral solubility could be used to estimate potential... 

The pancreatic digestion conditions studied included pH, the method of pH control, and bile salts mixture and concentration. In addition, experiments were run to determine if mineral solubility was affected by enzymatic activity, or only by pH-induced solubility changes. 

Fractionation Methods. Ultrafiltration and gel filtration are nondestructive methods which, based on limited experience, can be used for fractionation of mineral complexes from digests. In earlier studies mineral absorption on the gel material was a problem. Lonnerdal (30) introduced a method of treating dextran gels with sodium borohydride in order to eliminate the mineral-binding sites on the gel. In preliminary studies we have recovered more than 90 of Ca, Mg, Fe, Zn and P in samples applied to a borohydride-treated gel column (Sephadex G-50, Pharmacia Fine Chemicals, Piscataway, NJ). Recovery of Ca (Table IV) and Mg, Fe and Zn from ultrafiltration was also good. 

Elemental composition A1 52.91%, 0 47.08%. A1 may be anlayzed by atomic absorption or emission spectrophotometry or by colorimetric methods after acid digestion. Different forms of alumina may be identified by x-ray diffraction analysis. The X-ray crystallogaphic data for the mineral corundum are as follows ... 

Elemental composition Fe 46.55%, S 53.45%. The mineral may be characterized nondestructively by x-ray techniques. The compound may be analyzed for iron by AA or ICP/AES methods following digestion with nitric acid and appropriate dilution. 

Sample preparation methods are similar to those used for FAAS and ICP-AES. However, nitric acid is favoured for sample digestion since the other mineral acids contain elements which cause spectroscopic inteferences. 

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